Various known devices use images acquired by a sensor array, and correlation between images acquired by the sensor array, to determine deformations and/or displacements of an object. For example, one class of such devices is described in U.S. Pat. No. 6,873,422 (the '422 patent), U.S. Pat. No. 6,990,254 (the '254 patent), U.S. Pat. No. 6,996,291 (the '291 patent), and U.S. Pat. No. 7,065,258 (the '258) patent, all to Nahum, each of which is hereby incorporated by reference in its entirety. In general, in such devices, prior to displacing or deforming the object, a first or reference image arising from the object is captured and stored. Then, after displacing or deforming the object, a second or subsequent image arising from the object is captured and stored. The first and second images are then quantitatively compared, e.g. by correlation operations, on a pixel-by-pixel basis. In general, a plurality of respective comparisons are performed with the first and second images offset, or spatially translated, relative to each other by different respective amounts (e.g. by varying the offset in one pixel increments between the various comparisons). Then the resulting quantitative comparison, such as a correlation function value, is plotted against its corresponding offset amount, or spatial translation position, to determine a correlation function value point. The offsets having the strongest correlations between the second and first images will generate a peak or a trough (depending on how the pixel-by-pixel comparison is performed) in the plot of correlation function value points. The offset amount corresponding to the peak or trough represents the amount of displacement or deformation between the first and second images.
Various methods are known for estimating the peak or trough in a plot of correlation function value points with sub-pixel resolution. One of the more accurate methods is disclosed in the '422 patent, which describes how a sub-pixel error that is spatially periodic at the pixel pitch of the sensor array may arise in the correlation peak location estimates provided by various methods that use curve fitting. The '422 teaches various methods for reducing such errors when estimating a correlation peak location. However, some level of periodic sub-pixel error may remain after the methods of the '422 are applied. U.S. Pat. No. 7,085,431 to Jones (the '431 patent), which is hereby incorporated herein by reference in its entirety, teaches a method wherein the previously indicated periodic sub-pixel errors, and other errors, are characterized and compensated. However, there are calibration costs and signal processing complexity associated with the methods of the '431 patent.
The present invention is directed to a correlation peak finding method for image correlation displacement sensing that provides reduced spatially periodic sub-pixel errors while overcoming the foregoing and other disadvantages.